The aquaporins

Water is the major component of all living cells, and efficient regulation of water homeostasis is essential for many biological processes. The mechanism by which water passes through biological membranes was a matter of debate until the discovery of the aquaporin water channels. Aquaporins are intrinsic membrane proteins characterized by six transmembrane helices that selectively allow water or other small uncharged molecules to pass along the osmotic gradient. In addition, recent observations show that some aquaporins also facilitate the transport of volatile substances, such as carbon dioxide (CO(2)) and ammonia (NH(3)), across membranes. Aquaporins usually form tetramers, with each monomer defining a single pore. Aquaporin-related proteins are found in all organisms, from archaea to mammals. In both uni- and multicellular organisms, numerous isoforms have been identified that are differentially expressed and modified by post-translational processes, thus allowing fine-tuned tissue-specific osmoregulation. In mammals, aquaporins are involved in multiple physiological processes, including kidney and salivary gland function. They are associated with several clinical disorders, such as kidney dysfunction, loss of vision and brain edema

Funktionsanalyse pflanzlicher Aquaporine

Aquaporine zählen zur Superfamilie der MIP-homologen Proteine („major intrinsic proteins“). Sie bilden membranständige Poren, die die Passage von Wasser bzw. gelöster Stoffe wie zum Beispiel Glycerin und Harnstoff erleichtern. Der aquaporinvermittelte Stofftransport über Biomembranen ist nicht aktiver Natur, sondern folgt einem osmotischen Gradienten. Im Rahmen der vorliegenden Arbeit wurden drei Aspekte der Aquaporin-Funktion untersucht. Zunächst wurde die CO2-Permeabilität des Aquaporins NtAQP1 aus Nicotiana tabacum untersucht. Die aquaporinvermittelte zelluläre Aufnahme von CO2 konnte zunächst im heterologen Xenopus-Oozytenexpressionssystem nachgewiesen werden. Im Folgenden schlossen sich Untersuchungen im homologen pflanzlichen System an. Auch hier konnte eine veränderte zelluläre Aufnahme von CO2 in Abhängigkeit von der NtAQP1-Expression nachgewiesen werden. An intakten Pflanzen konnten darüber hinaus Effekte auf CO2-abhängige physiologische Vorgänge gezeigt werden. Die veränderte Expression von NtAQP1 beeinflusst den Mechanismus der Stomatabewegung und wirkt sich auf die Photosyntheseleistung der Pflanze aus. Aus den erhaltenen Ergebnissen kann abgeleitet werden, dass NtAQP1 im heterologen wie im homologen System unter anderem die Funktion einer CO2-Pore hat. Ein weiterer Teil der Arbeit beschäftigte sich mit der unterschiedlichen Wasserpermeabilität von PIP1- und PIP2-Aquaporinen am Beispiel zweier Vertreter aus Samanea saman. PIP1-Aquaporine induzieren in Xenopus-Oozyten in der Regel eine niedrige Wasserpermeabilität der Plasmamembran, während PIP2-Aquaporine deutlich höhere transmembrane Wasserflüsse ermöglichen. Mittels Gefrierbruch-Elektronenmikroskopie wurde die Dichte von Protein-Komplexen in den Plasmamembranen der Oozyten untersucht. Hierbei konnte nachgewiesen werden, dass die angesprochenen unterschiedlichen Wasserpermeabilitäten auf eine molekulare Eigenschaft des entsprechenden Proteins zurückgehen können und nicht auf einen unterschiedlichen Einbau der Proteine in die Plasmamembran beruhen. Als drittes Projekt wurde untersucht, inwiefern sich die Stickstoff-Versorgung von Phaseolus vulgaris in Form von Ammonium, bzw. Nitrat, auf die Aquaporinexpression in der Wurzel auswirkt. Kooperationspartner konnten nachweisen, dass die Versorgung von Wurzeln mit Ammonium als Stickstoffform eine verringerte Rate der Wasseraufnahme zur Folge hat. Im Rahmen der vorliegenden Arbeit konnte gezeigt werden, dass die Behandlung mit Ammonium zu einer reduzierten Expression von PIP1-Aquaporinen in der Wurzel führt

Aquaporins and plant leaf movements

BACKGROUND: Plant leaf movements can be mediated by specialized motor organs, the pulvini, or can be epinastic (i.e. based on different growth velocities of the adaxial and abaxial halves of the leaf). Both processes are associated with diurnally regulated increases in rates of membrane water transport, which in many cases has been shown to be facilitated by aquaporins. Rhythmic leaf movements are known from many plant species, but few papers deal with the involvement of aquaporins in such movements. SCOPE: Many details of the architecture and function of pulvini were worked out by Ruth Satter and co-workers using Samanea saman as a model organism. More recently a contribution of aquaporins to pulvinar movement in Samanea was demonstrated. Another model plant to study pulvinus-mediated leaf movements is Mimosa pudica. The contribution of both plasma membrane- and tonoplast-localized aquaporins to the seismonastic leaf movements in Mimosa was analysed. In tobacco, as an example of epinastic leaf movement, it was shown that a PIP1 aquaporin family member is an important component of the leaf movement mechanis

Aquaporins and membrane diffusion of CO2 in living organisms.

BACKGROUND Determination of CO2 diffusion rates in living cells revealed inconsistencies with existing models about the mechanisms of membrane gas transport. Mainly, these discrepancies exist in the determined CO2 diffusion rates of bio-membranes, which were orders of magnitudes below those for pure lipid bilayers or theoretical considerations as well as in the observation that membrane insertion of specific aquaporins was rescuing high CO2 transport rates. This effect was confirmed by functional aquaporin protein analysis in heterologous expression systems as well as in bacteria, plants and partly in mammals. SCOPE OF REVIEW This review summarizes the arguments in favor of and against aquaporin facilitated membrane diffusion of CO2 and reports about its importance for the physiology of living organisms. MAJOR CONCLUSIONS Most likely, the aquaporin tetramer forming an additional fifth pore is required for CO2 diffusion facilitation. Aquaporin tetramer formation, membrane integration and disintegration could provide a mechanism for regulation of cellular CO2 exchange. The physiological importance of aquaporin mediated CO2 membrane diffusion could be shown for plants and cyanobacteria and partly for mammals. GENERAL SIGNIFICANCE Taking the mentioned results into account, consequences for our current picture of cell membrane transport emerge. It appears that in some or many instances, membranes might not be as permeable as it was suggested by current bio-membrane models, opening an additional way of controlling the cellular influx or efflux of volatile substances like CO2. This article is part of a Special Issue entitled Aquaporins

Characterization of two tomato aquaporins and expression during the incompatible interaction of tomato with the plant parasite Cuscuta reflexa.

A subtractive suppression hybridization technique was used to identify genes that were induced during early phases of the interaction between Cuscuta reflexa, a phanerogamic plant parasite and the incompatible host tomato (Lycopersicon esculentum Mill.). One of the identified genes encodes a new aquaporin (LeAqp2) from tomato. Its function was concluded from the swelling kinetics of LeAqp2-expressing Xenopus laevis oocytes under hypo-osmotic conditions. It was shown that, 6 h after attachment of the plant parasite, the corresponding mRNA accumulated in cells at and adjacent to the attachment site of Cuscuta, while artificial wounding did not modify steady-state LeAqp2- RNA levels. Expression of a close homologue named TRAMP (tomato-ripening-associated protein) was not affected by the plant-plant interaction. Levels of indole-3-acetic acid (IAA) in tomato tissue after infection by Cuscuta have been found to increase at a similar stage of infection. In contrast to the different behavior with respect to infection, IAA induced both LeAqp2 and TRAMP expression. The observed pattern of LeAqp2 expression during the interaction at a stage where cell elongation occurs together with the water-channel activity in the heterologous expression system suggest a function for LeAqp2 during the tomato-Cuscuta interaction

The Arabidopsis aquaporin PIP1;2 rules cellular CO(2) uptake.

The membrane CO(2) flux into Arabidopsis mesophyll cells was studied using a scanning pH microelectrode. Arabidopsis thaliana mesophyll cells were exposed to photosynthesis-triggering light intensities, which induced cellular CO(2) uptake. Data obtained on a AtPIP1;2 T-DNA insertion line indicated that under these conditions, cellular CO(2) transport was not limited by unstirred layer effects but was dependent on the expression of the aquaporin AtPIP1;2. Complementation of the AtPIP1;2 knockout restored membrane CO(2) transport levels to that of controls. The results provide new arguments for the ongoing debate about the validity of the lipid bilayer model system and the Meyer-Overton rule for cellular gas transport. In conclusion, we suggest a modified model of molecular gas transport mechanisms in living cells. © 2011 Blackwell Publishing Ltd